Various hydrogen absorbing alloy application systems, such as heat pumps, hydrogen storage systems and fuel cell systems, have been developed which utilize the hydrogen storage function or thermal energy conversion function of hydrogen absorbing alloys.
With such systems it is the practice to pulverize to a powder, an ingot of hydrogen absorbing alloy which has been obtained by melting, fill the powder into a container of specified capacity and cause the alloy to abrorb and desorb hydrogen. In this case, it is desired to pack the container with a large amount of the alloy by diminishing the voids within the container to the greatest possible extent. However, the powder obtained by pulverizing the ingot of hydrogen absorbing alloy has irregularly shaped particle surfaces, includes many voids among the particles and is therefore difficult to pack into the container to a high density. More specifically stated, in the case where particles of approximately same size are packed into a container of specified capacity, the volume ratio of the voids formed inside the container per unit volume thereof, i.e., void fraction, is difficult to decrease to below 0.5 which is the limit value of void fraction when the particles are assumed to be spherical, because the particles are not spherical.
Since the particles of hydrogen absorbing alloy repeatedly expands and contracts with the absorption and desorption of hydrogen, the resulting stress acts on the wall of the container, possibly deforming the container. Especially when the alloy particles are more finely divided and scatter during the repeated expansion and contraction, the fine particles progress toward, and concentrate in the bottom of the container, almost eliminating the interstices between the alloy particles in the bottom portion of the container . As a result, the expansion of the alloy in this area acts directly on the container wall, deforming the container and possbily causing a break in the wall. This gives rise to the problem of so-called "swelling." Accordingly, the powder of the hydrogen absorbing alloy is conventionally pelletized, as will be described below, to cope with this problem.
According to a first method of pelletization, a polymeric material is mixed with the alloy powder, and the mixture is pelletized by heating (JP-B-18521/1981, JP-A-147032/1984, JP-A-119501/1989 and JP-A-246101/1989). For example, JP-B-18521 discloses a pelletizing method wherein a viscoelastic substance comprising a polymeric material is admixed with the alloy powder, and the mixture is enclosed with a covering material of porous plate, followed by sintering. Another method is known wherein an elastic material comprising a high polymer is admixed with the alloy powder, and the mixture is packed into shells of porous material for pelletization (JP-A-83901/1984).
According to a second method, a ceramic is admixed with the alloy powder, and the mixture is pelletized as by sintering (JP-A-158101/1980, JP-A-209901/1986, JP-A-73401/1984 and JP-A-38302/1982).
A third method comprises admixing aluminum or like metal with the alloy powder and pelletizing the mixture as by sintering (JP-B-29921/1980, JP-A-109802/1981, JP-B-4321/1987, JP-A-79701/1988 and JP-A-112401/1988). For example, JP-B-29921/1980 uses Al, Sn, Zn or the like as a binder, while JP-A-109802/1981 and JP-B-4321/1987 use Al, Ni, Cu or like metal as a binder.
However, the pelletization of the first to third methods described requires a heat treatment or packing of the powder into porous shells or the like and therefore has the problem of making the production process complex. Furthermore, the presence of the polymeric material or like binder causes the problem of greatly reducing the ratio (packing fraction) of the hydrogen absorbing alloy in the pellets to below 50% which is the standard packing fraction for conventional containers packed with the alloy powder without the polymeric material. For example, with the pellets of JP-A-246101/1989 wherein CaNi.sub.5 is used as the hydrogen absorbing alloy, and a phenolic resin or fluorocarbon resin is used as the polymeric material, the density (true density) of the alloy itself is about 6.6 g/cm.sup.3, while the mass (bulk density) of the pellets including interstices per unit volume thereof is 4.12 g/cm.sup.3 at its highest. This value is less than 63% of the true density of the alloy particles. Moreover, the volume ratio of the interstices (porosity) is above 26%.